The present invention relates to a method and, means for achieving good measuring accuracy in pulsed optical distance meters of transit-time type. By this is meant distance meters comprising an optical sender, preferably a laser, emitting very short optical pulses. When the optical pulses encounter the object to be measured, a part of the radiation is reflected to an optical receiver which is built into the distance meter. The delay occuring between the emitted and the received optical pulse serves as a measurement of the distance between distance meter and measured object.
In distance meters of the type described it has long been customary to use an electronic clock consisting of a crystal-controlled oscillator, and a counter which is successively stepped forward by the oscillator, to measure the time between emitted and received pulse. This counter, which is initially set at zero, is started when the optical pulse is emitted and stopped when the reflected pulse is received. In order to achieve good measuring accuracy the counter must permit time measurement with good resolution, and this in turn demands a high-frequency oscillator. Furthermore, it must be possible to determine the time at which the pulse is received with good reproduceability. The optical receiver generally includes a photo-detector and an amplifier with band pass characteristic. The band width of the amplifier is normally adjusted to give prominence to the signal pulse received as far as possible over noise and interference. With this type of filtering the electrical output signal from the optical receiver will consist of pulses whose rise and fall times are of the same order of magnitude as the pulse length measured at half the amplitude value. A binary signal representing a logical one is required as stop signal for the counter at the time when a pulse is received, as well as a logical zero before. Such a signal is usually effected by the output pulse from the receiver being supplied to a threshold circuit arranged to emit a logical one when the signal amplitude from the optical receiver exceeds a certain predetermined value, but otherwise to emit a logical zero. The uncertainty which is therefore obtained in the time-determination is dependent on the variations in signal amplitude obtained due to varying distance to the object being measured and due to atmospheric conditions. If the signal has low amplitude the predetermined threshold level will be exceeded at an early stage of the pulse's duration whereas if the signal has high amplitude the threshold will only be exceeded close to maximum amplitude at the mid-point in time of the pulse. This means that a measurement-uncertainty share is obtained which is of the same order of magnitude as the rise time of the pulse. Thus, in a distance meter with high measuring accuracy, an extremely high oscillator frequency is required and the pulses emitted must also be very short. In the pulsed laser distance meter currently in use, the half-value width of the pulse length is usually 30-60 nanoseconds and the oscillator frequency is usually 14,990 or 29,979 megaherz, which gives a measuring resolution of 10 and 5 meter, respectively. The most modern technology now enables shorter pulses and considerably higher oscillator frequencies but such technology is complicated both from the design and the manufacturing point of view, thus making such distance meters expensive.
Swedish patent No. 8 605 335-2, for instance, describes a technique for circumventing the problems described above. Characteristic of the invention according to said patent is that both the pulse emitted and that received are transmitted via analog-digital converters to series of binary numbers which are stored in shift registers or similar memories. By shifting the contents in one of the shift registers step by step once storage has been performed, and by counting the correlation coefficient between the contents in the two shift registers at each step, a position can be found where the correlation coefficient has its greatest value. The number of steps required to reach this position constitutes a measurement of the delay between the pulse emitted and that received. If the correlation optimum happens to be between two steps it can be determined by interpolation. A tenfold improvement of the measuring accuracy can easily be achieved by using the invention according to this patent. However, devices accordance to said patent have the drawback that they require a vast amount of electronic equipment and measuring the distance is time-consuming because of the large number of counting operations required to determine correlation, and this in turn limits how often measurement of the distance can be repeated.